Fossils found in the Czech Republic are the only known remains in Europe that date to the period when Neanderthals are thought to have become extinct. The skulls are considered "modern" but have "archaic" features. (Wolfgang Reichmann)

For decades, scientists have argued over the disappearance of Neanderthals from prehistoric Europe about 30,000 years ago. Did they die from some mysterious disease? Or did modern humans simply supplant them, either by obliterating them or by interbreeding?

In research reported today in the journal Nature, an Austrian-led team has added fuel to the debate, confirming that fossil remains from a famous archaeological site in the Czech Republic are 31,000 years old -- putting them at the period when Neanderthals vanished.

The bones from the Mladec Caves represent the only known remains in Europe that can be linked directly to "Aurignacian" stone and bone tools, ornaments, and other artifacts made 30,000 to 40,000 years ago, when humans first began to fashion objects with aesthetic as well as utilitarian purposes.

While the bones -- from six individuals found in the caves -- are generally regarded as "modern," some of the fossil skulls show "archaic" features, among them heavy brow ridges and protruding bone in the back of the head, that are more associated with Neanderthals.

"These characteristics could be explained by interbreeding, or seen as Neanderthal ancestry," team leader Eva Maria Wild of the University of Vienna said in an e-mail. "The finds are essential in the ongoing debate over the emergence of modern humans in Europe. The discussions will continue."

Modern humans arrived in Europe about 40,000 years ago, from Africa via the Middle East. By 27,000 years ago they had completely replaced the ice-age Neanderthals, who had lived in Europe for at least 100,000 years and became extinct for unknown reasons.

Until recently, much of the understanding of modern humans' early sojourn in Europe was based on a small number of fossil remains, among them those from Mladec, along with a much larger number of archaeological sites containing items such as flint chopping tools, exquisitely carved knives and elaborate cave paintings. Most of the sites were discovered decades, or even centuries, ago. Mladec was found in the early 19th century and systematically excavated in 1881.

In recent years, however, modern dating techniques have shown that the bones in most of these Aurignacian sites are much younger than the artifacts, calling into question the link between modern humans and their supposed tools.

The Wild team's work was the latest research to reexamine the early conclusions. Wild said the team used radiocarbon analysis on teeth and one bone. The results made Mladec the only known Aurignacian artifact site with human remains from the same period.

"It's nice to know that at least one of the sites matches the bones," said paleoanthropologist Ian Tattersall of the American Museum of Natural History.

Anthropologist Erik Trinkaus of Washington University in St. Louis, a member of the Wild team and a leading exponent of the Neanderthal disappearance-through-interbreeding school, said the new findings have thrown the debate over Neanderthals' fate "into a jumble."

"Either there's been an evolutionary leveling, or there has been some level of interbreeding, and we will never know how much," Trinkaus said. "My answer is, why not [interbreeding]? They were all dirty and smelly, and didn't have much opportunity" for social activity, he said.

But Tattersall, a leading opponent of the interbreeding theory, said the Mladec remains were "perfectly routine Homo sapiens [modern humans]. The only people who believe otherwise are those with an ax to grind."

Still, Trinkaus, Tattersall and other scholars acknowledged that the new findings were unlikely to settle the debate. "I think most people believe [the Mladec remains] are Homo sapiens, " said University of Arizona archaeologist Steven Kuhn.

"But while the majority believe that the Neanderthals did not have a huge input, they can't rule out some input" in the development of modern Europeans, he added. "And a little is more than zero."